Surface acoustic wave (SAW) devices use waves propagated on the surface of an elastic solid for electronic signal processing. A typical SAW device uses a transducer to convert electromagnetic signal waves, which travel at the speed of light, to acoustic signal waves, which travel at speeds on the order of 10.sup.5 less than the speed of light. This substantial reduction in wave length allows designers to implement certain complex signaling processing functions in a significantly smaller space than would be needed for traditional circuit designs. Thus, a SAW device designed to handle complex signal processing functions can offer considerable cost and size advantages over competing technologies. SAW technology is increasingly found in applications such as filters, resonators, oscillators, delay lines, and other similar devices.
SAW devices are typically implemented on a piezoelectric substrate and usually employ interdigital transducers (IDTs) located on the surface of the piezoelectric substrate to generate and detect acoustic waves. The geometry of the IDTs (beam width, pitch, number of fingers) on the piezoelectric substrate plays a significant role in the signal processing and frequency response characteristics of a SAW device. SAW device designers generally achieve the desired operating frequency response of the device by focusing on the geometry of the IDTs, and by the choice of materials used for the piezoelectric substrate.
Conventional SAW filters have transducers located side by side on common tracks and use acoustic coupling to couple between the transducers, however, each acoustic track must be tuned for the same resonant operating frequency. Historically, SAW devices have had problems with insertion loss with a typical insertion loss for SAW filters being greater than 3.5 dB. One of the large loss mechanisms in today's high coupling coefficient SAW filters is the acoustic wave attenuation. The acoustic attenuation is the amount of energy lost or converted into unrecoverable bulk energy as the surface wave propagates along the surface of the piezoelectric substrate.
An improvement over the conventional SAW filter is the SAW ladder filter. Conventional SAW ladder filters are differentiated from conventional non-ladder SAW filters in that the transducers in the ladder design are acoustically staggered, use only electrical coupling, and operate at different frequencies (resonant and anti resonant) at a center frequency .function..sub.0.
Referring now to FIGS. 1 and 2 of the accompanying drawings there is shown a prior art SAW ladder filter 100 and its' associated equivalent circuit model 200. The filter 100 includes 7 resonators 150-162 located on 7 separate acoustic tracks of a piezoelectric substrate 101. Each resonator 150-162 is comprised of a transducer 102-114 and two reflectors 116-142. The resonators 150-162 are electrically coupled through their respective transducers at nodes 1 to 5. Conventional SAW ladder filter designs have their acoustic paths in different propagation paths so the acoustic energy leaving the end of a transducer does not interfere with the response of another transducer. These SAW ladder filters can be designed with and without the reflectors. However, the acoustic energy leaving the transducer in a non-reflector design generally creates a large loss mechanism in the filter.
The purpose of the reflector is to conserve the energy being lost out the ends of the transducer by reflecting the acoustic energy back into the transducer which increases the resonator's unloaded Q (Q.sub.u). The reflector, however, is not an ideal device, as acoustic energy is lost in the reflector. The energy lost in the reflector is due to the acoustic attenuation of the surface wave as it travels into the reflector and then back out. The acoustic attenuation has two components. The first component being gradual conversion of the surface mode to bulk mode as it travels on the uniform surface. The second component being the acoustic scattering that takes place when a surface wave hits a discontinuity, such as the reflector fingers. The entire time the surface wave is in the reflector, no energy is being utilized by the transducer, and consequently, the reflector loss degrades the filter insertion loss.
While an improvement over conventional non ladder filters, the typical insertion loss for ladder filter devices is still greater than 2.5 dB. A drawback to conventional SAW ladder designs is a strict impedance requirement which forces narrow constraints on the beam width and pitch of the individual transducers. Also, SAW ladder filters have historically needed a large surface area for implementation because of the separation of all the transducers and the additional reflectors.
Hence, there is a need for an improved SAW device that minimizes acoustic losses and provides improved filter performance, particularly with regards to insertion loss, while reducing the surface area required in order to be implemented.